Methods of growing nitride-based film using varying pulses

ABSTRACT

Nitride-based film is grown using multiple precursor fluxes. Each precursor flux is pulsed one or more times to add a desired element to the nitride-based film at a desired time. The quantity, duration, timing, and/or shape of the pulses is customized for each element to assist in generating a high quality nitride-based film.

REFERENCE TO RELATED APPLICATIONS

The current application is a divisional application of co-pending U.S.patent application Ser. No. 10/713,326 filed on Nov. 14, 2003, whichclaims priority to provisional application Ser. No. 60/468,516, filed onMay 7, 2003, both of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates generally to epitaxial growth of semiconductormaterials, and more specifically to methods for growing nitride-basedfilm using varying pulses.

2. Related Art

Typically, nitride-based films (e.g., heterostructures) are grown usingeither Metal Organic Chemical Vapor Deposition (MOCVD), Molecular BeamEpitaxy (MBE), or Reactive Molecular-Beam Epitaxy (RMBE). MOCVD requiresa relatively high growth temperature that makes it difficult to grownitride-based films that include Indium (In) and/or Aluminum (Al). WhileMBE and RMBE allow growth at lower substrate temperatures, they aredifficult to adopt for growing high quality insulating nitride films.

Current optoelectronic nitride devices require about twenty-eightpercent Al to operate at wavelengths of approximately three hundrednanometers. For example, an optoelectronic nitride device can include anAlGaN (Aluminum-Gallium-Nitrogen) layer grown over a GaN buffer layer.Due to the lattice mismatch between the two layers, current depositiontechniques cannot create a reliable AlGaN layer having a thicknessgreater than approximately one hundred fifty Angstroms. As a result,currently attainable thicknesses are insufficient for fabricatingvarious semiconductor devices, such as a photodetector or a multiplequantum well light-emitting diode that operates at wavelengths ofapproximately three hundred nanometers.

Including In in one or both of the AlGaN and GaN layers has been shownto provide greater control over lattice mismatch and strain in thestructure. The greater control allows for the creation of devices thatare more reliable. A Pulsed Atomic Layer Epitaxy (PALE) technique wasdeveloped to deposit AlInGaN layers. PALE layer growth uses a series ofmetal organic precursor flux pulses to deposit the desired elements.

FIG. 1 shows an illustrative sequence of precursor flux pulses thatgenerate an AlInGaN layer using the PALE technique. The sequenceincludes several series of pulses 10, 12, 14, 16. Series of pulses 10applies Al using a precursor flux comprising Tri Methyl Aluminum (TMA),series of pulses 12 applies N using a precursor flux comprising NH₃,series of pulses 14 applies In using a precursor flux comprising TriMethyl Indium (TMI), and series of pulses 16 applies Ga using aprecursor flux comprising Tri Methyl Gallium (TMG). Each series ofpulses 10, 12, 14, 16 includes at least one pulse. In the sequence, onlyone precursor flux is pulsed at any given time 18. Each precursor fluxflows at a constant rate during a pulse, and does not flow when notpulsed. As a result, each pulse is shown having a rectangular shape. Theduration of each pulse was set at approximately six seconds. In thesequence, series of pulses 12 is pulsed every other time, with one ofthe metalorganic precursor fluxes 10, 14, 16 pulsing between eachnitrogen precursor flux pulse.

While the PALE technique allows for the deposition of high qualityepitaxial layers at temperatures close to those used by MBE and RMBE, itprovides a relatively slow growth rate and does not incorporate anyflexibility to allow for optimizing growth of the nitride-based films.

As a result, a need exists for improved methods of growing anitride-based film that are more efficient and more flexible to producehigher quality, larger nitride-based films.

SUMMARY OF THE INVENTION

The invention provides improved methods of growing a nitride-based filmusing varying precursor flux pulses. In particular, the quantity,timing, duration, and/or shape of each precursor flux pulse is alteredbased on the corresponding element to provide a higher qualitynitride-based film. As a result, additional control over the growth ofthe nitride-based film is obtained.

A first aspect of the invention provides a method of growing anitride-based film, the method comprising: applying a first precursorflux for a first element using a first pulse, wherein the first pulsehas a first duration and wherein the first element comprises nitrogen;and applying a second precursor flux for a second element using a secondpulse, wherein the second pulse has a second duration, and wherein thesecond duration is not equal to the first duration.

A second aspect of the invention provides a method of growing anitride-based film, the method comprising: applying a first precursorflux for a first element using a first series of pulses, wherein thefirst element comprises nitrogen; and applying a second precursor fluxfor a second element using a second series of pulses, wherein at least aportion of a pulse in the second series of pulses is applied during atleast a portion of a pulse in the first series of pulses.

A third aspect of the invention provides a method of growing anitride-based film, the method comprising: applying a nitrogen precursorflux comprising NH₃ using a first series of pulses; and applying asecond precursor flux for a second element using a second series ofpulses, wherein a pulse in the second series of pulses has anon-rectangular waveform.

The illustrative aspects of the present invention are designed to solvethe problems herein described and other problems not discussed, whichare discoverable by a skilled artisan.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is an illustrative sequence of flux pulses used in the prior artPALE technique;

FIG. 2 is an illustrative sequence of flux pulses according to oneembodiment of the invention;

FIG. 3 is an illustrative sequence of flux pulses according to a secondembodiment of the invention;

FIG. 4 is an illustrative sequence of flux pulses according to a thirdembodiment of the invention;

FIG. 5 shows normalized photoluminescence values for an InN layer grownusing the conventional MOCVD technique and an InN layer grown using oneembodiment of the invention; and

FIG. 6 shows an illustrative substrate with silicon dioxide stripes.

It is noted that the drawings of the invention are not to scale. Thedrawings are intended to depict only typical aspects of the invention,and therefore should not be considered as limiting the scope of theinvention. In the drawings, like numbering represents like elementsbetween the drawings.

DETAILED DESCRIPTION OF THE INVENTION

It is understood, that for purposes of this description Al meansAluminum, Ga means Gallium, N means Nitrogen, In means Indium, Si meansSilicon, Mg means Magnesium, Mn means Manganese, C means Carbon, and Hmeans Hydrogen.

As discussed above, the invention provides improved methods of growing anitride-based film using varying precursor flux pulses. In particular,the quantity, timing, duration, and/or shape of each precursor fluxpulse is altered based on the corresponding element to provide a higherquality nitride-based film. As a result, additional control over thegrowth of the nitride-based film is obtained.

Turning to the Figures, FIG. 2 shows several series of pulses in whicheach series of pulses 20, 22, 24, 26 is used to apply a unique precursorflux onto a nitride-based film that is being grown. Series of pulses 20applies an Al precursor flux comprising TMA, series of pulses 22 appliesa N precursor flux comprising NH₃, series of pulses 24 applies an Inprecursor flux comprising TMI, and series of pulses 26 applies a Gaprecursor flux comprising TMG. Each series of pulses 20, 22, 24, 26includes one or more pulses. In this embodiment, the nitrogen precursorflux series of pulses 22 is pulsed every other time over the sequence,and no two pulses occur at the same time. The duration of pulses isadjusted based on the precursor flux being pulsed to allow for improvedsurface migration of the precursor flux at the growth surface. This isachieved because different species have different surface migrationrates that are accommodated by the invention. For example, series ofpulses 22 includes multiple pulses 22A-C that have the same duration 28.Series of pulses 20 comprises two pulses 20A, 20B that also have thesame duration 28. However, series of pulses 24 includes two pulses 24A,24B that have a duration 30 that is less than duration 28, and series ofpulses 26 includes a single pulse 26A that has a duration 32 that isgreater than duration 28.

Series of pulses 22 also includes gaps 34A-C between consecutive pulses22A-C. Gap 34A has the same duration 28 as the pulses 22A-C. Theduration 28 of gaps 34A-C can be maintained at constant duration 28, oraltered based on the duration of an intermediate pulse being applied forone of the other series of pulses 20, 24, 26. For example, gap 34B has aduration 32 that is the same as the duration for pulse 26A. As a result,the gap 34B between the pulses 22B-C that respectively occur immediatelybefore and after pulse 26A has a longer duration 32 than other gaps 34A,34C in the series of pulses 22. However, gap 34C has a duration that islonger than duration 30 for pulse 24A. The duration of gap 34C could bethe same as duration 28 for gap 34A and/or other gaps in series ofpulses 22.

FIG. 3 shows several alternative series of pulses in which each seriesof pulses 40, 42, 44, 46 is used to apply a unique precursor flux onto anitride-based film that is being grown. As in FIG. 3, a nitrogenprecursor flux series of pulses 42 is pulsed every other time over thesequence. However, portions of the pulses for the remaining series ofpulses 40, 44, 46 occur at the same time as one of the pulses in thenitrogen precursor flux series of pulses 42. For example, pulse 40Astarts before pulse 42A ends, pulse 44A starts before pulse 42B ends,and pulse 46A continues through the start of pulse 42C. It is understoodthat numerous variations are possible. For example, two or more pulsescan simultaneously start and/or finish, a pulse can start and finishduring one or more other pulses, etc. Further, portions of a pulse canoverlap the end of a previously started pulse and the start of asubsequently started pulse. Use of overlapping pulses providesadditional ability to optimize the structure being grown and providessmoother transitions between stages than those provided by conventionalMOCVD and PALE. For example, the overlapping pulses allow for a moregradual transition between the layers having different composition. Inthis case, the degree of the overlap can be used to control thesharpness of the transition.

FIG. 4 shows another alternative embodiment of several series of pulsesin which each series of pulses 60, 62, 64, 66 is used to apply a uniqueprecursor flux onto a nitride-based film that is being grown. In thisembodiment, the waveforms for each pulse 60A in series of pulses 60 isaltered from the rectangular shape used in the previous embodiments. Themodified waveform represents a flow rate for the particular precursorflux that varies during the pulse. In the related art, the precursorflux was either flowing at a constant rate or not flowing at all. Use ofaltered waveforms provides additional control over the migration of theprecursor flux at the growth surface to help ensure a more uniformmigration. Since the migration rate is a function of the surfacecoverage, the tailored waveforms allow for the migration rate to beoptimized at all stages of the surface coverage. FIG. 4 also showspulses 62A-C having a different non-rectangular waveform for optimizingthe nitrogen precursor flux, pulses 64A having a shortened duration witha rectangular waveform, and pulse 66A having a longer duration thatoverlaps the end of pulse 62B and the start of pulse 62C.

It is understood that each of the series of pulses depicted in FIGS. 2-4are only illustrative of the present invention. The invention is notlimited to particular elements or types of precursor flux, a specificquantity of series of pulses, any duration, quantity, or pattern ofpulses in each of the series of pulses, etc. Further, various featuresof each of the series of pulses can be combined to form a desiredsequence of pulses for elements being deposited on a film. For example,pulse duration and shape can be modified for each pulse in a series ofpulses based on the desired attributes of a structure.

The various sequences of pulses provide better incorporation of atomsinto the growing crystal and improved surface coverage by providingbetter mobility of the pre-cursor species on the growth surface. As aresult, the epitaxial growth technology has been shown to scale up toapproximately four inch structures and provide a large reduction ofadverse aging effects. Further, the invention allows for sharpnitride-based film heterostructures, improved quality of quantum wellsand barriers, better incorporation of In into AlGaN, and for theinclusion of thin (i.e., a few monolayers to a few nanometers) smoothinglayers for strain control and a resulting reduction in growth defects.Still further, the invention allows a nitride-based film to be grown atlower temperatures. Because of the lower temperature, the nitride-basedfilm can be grown over substrates comprising material such as lithiumaluminate, lithium gallate, diamond, silicon, or the like that areunstable at the temperatures required using previous approaches. As willbe recognized by one skilled in the art, products created using thepresent invention can be used in many types of semiconductor devices,including power switching devices, and light emitting devices.

For a comparison of growth techniques, a layer was grown using anexisting technique and another layer was grown using one embodiment ofthe invention. In particular, both layers were grown on a 1.5 mm thicksemi-insulating GaN layer. Using the conventional method, the GaN layerwas deposited on a low temperature AlN buffer layer grown on a c-planesapphire substrate. Subsequently, an InN layer was grown on the GaNlayer using the conventional MOCVD method, and another InN layer wasgrown on the GaN layer using one embodiment of the invention. In theembodiment used, an InN epitaxial layer was grown at 550° C. with N₂ asa carrier gas. During the InN growth, the durations of alternating TMIpulses and NH₃ pulses were both set at six seconds. However, thealternating pulses overlapped for a period of two seconds, creating arepeating flow of In (four seconds), InN (two seconds), and N (fourseconds). A total of six hundred pulses were used to grow the layer,which was approximately 0.3 mm thick. It is understood thatcorrespondingly more/less pulses could be used for growingthicker/thinner layers.

FIG. 5 shows the results of a comparison of normalized photoluminescence(PL) values for the two layers. In particular, the PL intensity(arbitrary units) is plotted as a function of energy (eV) for the layergrown using the above-described embodiment of the invention and thelayer grown using the conventional MOCVD method. As can be seen, the PLintensity 80 for the layer grown using the invention is considerablyhigher than the PL intensity 82 for the layer grown using theconventional MOCVD method. This shows that the growth technique of theinvention can yield high quality layers.

Additional features can be incorporated along with the varied precursorflux pulses to further improve and/or control growth. For example, thenitride-based film can be illuminated with ultraviolet radiation. Theillumination can occur during a particular pulse, between pulses, orboth. Illumination of the growth surface with ultraviolet radiation canactivate the precursor gases (if during a pulse) and/or activate agrowth species at the growth surface (if between pulses). Further, oneor more additional pulses can be incorporated during and/or between thevarious pulses. For example, a constant background precursor flux can beapplied during the series of pulses. Additionally, doping pulses can beincorporated to modulate doping during the deposit of one or morelayers. To this extent, dopants such as Si, Mg, Mn, C, or the like canbe deposited using one or more pulses.

Other aspects of the growth environment can also be varied to improveand/or control growth. For example, a growth temperature can be variedto further improve and/or control growth. The growth temperature can bevaried gradually or abruptly and/or during a single pulse or overseveral pulses. By varying the growth temperature, additional controlover the quality of the material, composition and strain profile of thematerial, and/or growth and lateral overgrowth rates and conditions canbe obtained. Similarly, the pressure can be varied during the depositionof one or more layers to provide an additional degree of control.

Various additional aspects can be incorporated into the substrate onwhich one or more layers are grown using the invention. For example,FIG. 6 shows a substrate 70 having a series of silicon dioxide stripes72A-D deposited thereon. The invention can be used for lateralovergrowth epitaxy, in which one or more nitride-based films can bedeposited over substrate 70 and/or silicon dioxide stripes 72A-D. It isunderstood, however, that any material and/or pattern (e.g., stripes,stars, etc.) can be deposited on substrate 70. Further, the inventioncan be used for the lateral air-bridged deposition of nitride-basedfilms over various substrates, including a patterned substrate ofwurtzite symmetry used for growth over a silicon substrate, a polarsubstrate having a wurtzite crystal symmetry and patterned with polartrenches (C-plane), a non-polar substrate patterned with trenchescomprising sapphire, SiC, Si, bulk GaN, bulk AlN, bulk AlGaN, lithiumaluminates, or the like.

The invention can be used in numerous applications. For example, lateralgrowth of a material such as GaN, AlN, InN, AlGaN, AlInGaN, or the likecan also be performed using the invention in order to obtain an improvedquality for the material over current solutions. For example, a templatelayer of the material that includes one or more “holes” can be depositedover a substrate comprised of sapphire, SiC, Si, diamond, or the like,using a wet etch to decorate defects concentrated near threadingdislocations. The invention can then be used for lateral overgrowth ofthe holes in the template layer.

Further, a plurality of low-temperature (low-T) islands/nucleationcenters comprising the material can be deposited on SiC substrates orthe like. The invention can then be used in the lateral growth of ahigh-temperature (high-T) layer of the material. In this case, the low-Tislands can act as a selective area deposited material without the useof a masking layer. By appropriately configuring a time intervalbetween, for example, a Ga and ammonia pulse and an Al and ammonia pulseduring growth of AlGaN, separation due to varying Ga and Al surfacemobility should be minimized and/or prevented.

Various other applications are also possible. For example, the inventioncan be used to grow anisotropic conductivity structures or the like.Additionally, a doping superlattice can be grown using the invention tocontrol strain in a device. Still further, doping control and/or spatialin-plane composition can be performed by non-uniform species migrationand/or non-uniform precursor flows. The invention can also be used toperform co-doping by native species.

The foregoing description of various embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof the invention as defined by the accompanying claims.

1. A method of growing a nitride-based film, the method comprising: applying a first precursor flux for a first element using a first series of pulses, wherein the first element comprises nitrogen; and applying a second precursor flux for a second element using a second series of pulses, wherein at least a portion of a pulse in the second series of pulses is applied during at least a portion of a pulse in the first series of pulses.
 2. The method of claim 1, further comprising applying a third precursor flux for a third element using a third series of pulses, wherein at least a portion of a pulse in the third series of pulses is applied during at least a portion of a pulse in the first series of pulses.
 3. The method of claim 2, further comprising applying a fourth precursor flux for a fourth element using a fourth series of pulses, wherein at least a portion of a pulse in the fourth series of pulses is applied during at least a portion of a pulse in the first series of pulses.
 4. The method of claim 1, further comprising illuminating the nitride-based film with ultraviolet radiation.
 5. The method of claim 1, wherein a pulse in the second series of pulses has a non-rectangular waveform.
 6. The method of claim 1, wherein the first precursor flux comprises NH₃, and the second precursor flux comprises at least one of: TMG, TMI, or TMA.
 7. The method of claim 1, wherein the nitride-based film is grown on a substrate comprising at least one of: lithium aluminate or silicon.
 8. The method of claim 1, further comprising varying a growth temperature for the nitride-based film.
 9. The method of claim 1, further comprising varying a growth pressure for the nitride-based film.
 10. The method of claim 1, further comprising applying a doping precursor flux for a dopant using a third series of pulses.
 11. A method of growing a nitride-based film, the method comprising: applying a first precursor flux for a first element using a first series of pulses; and applying a second precursor flux for a second element using a second series of pulses, wherein at least a portion of a pulse in the second series of pulses is applied during at least a portion of a pulse in the first series of pulses, wherein the first and second elements each comprise a unique element selected from the group consisting of: nitrogen, aluminum, gallium, and indium.
 12. The method of claim 11, further comprising illuminating the nitride-based film with ultraviolet radiation.
 13. The method of claim 11, further comprising varying a growth temperature for the nitride-based film.
 14. The method of claim 11, further comprising varying a growth pressure for the nitride-based film.
 15. The method of claim 11, further comprising applying a doping precursor flux for a dopant using a third series of pulses.
 16. A method of growing a nitride-based film, the method comprising the sequential steps of: flowing a first precursor flux for a first element; simultaneously flowing the first precursor flux and a second precursor flux for a second element; and flowing the second precursor flux, wherein one of: the first element or the second element comprises nitrogen and the other of the first element or the second element does not comprise nitrogen.
 17. The method of claim 16, further comprising repeating the flowing, simultaneously flowing, and flowing steps a plurality of instances.
 18. The method of claim 16, wherein the first element comprises nitrogen, the method further comprising the sequential steps of: flowing the first precursor flux; simultaneously flowing the first precursor flux and a third precursor flux for a third element; and flowing the third precursor flux, wherein the third element does not comprise nitrogen and is different from the second element.
 19. The method of claim 18, wherein the second element and the third element each comprise a unique element selected from the group consisting of: aluminum, gallium, and indium.
 20. The method of claim 16, further comprising varying at least one of: a growth temperature or a growth pressure for the nitride-based film. 